Continuing along the lines of feedback, I'd suggest highway radar/display signs that show the average speed of vehicles on the highway.
Rationale: in Lake Wobegon, everyone drives faster than the average. I think there's a perception bias. First, I'd assume that the distribution of vehicle speeds on a highway is a skewed normal distribution, that is, there are very few vehicles going at or below the speed limit, and significant numbers going well above the speed limit. The peak of the curve is somewhere above the limit.
Second, assume you merge onto the highway and drive at your normal speed, maybe 65 mph in a 55 mph zone. You're going to be passed by vehicles going 70, 80 and 90; you're going to pass vehicles going 55. I suspect the speeders are going to make the biggest impact on you. You may well believe that the average speed is 70. But that may not be true. A display setup would show the truth.
(This idea is a takeoff from some research that supposedly showed that college students believe binge drinking is more prevalent than it actually is, resulting in a greater tendency to drink. I say "supposedly" because I've also seen a challenge to the research and don't know for sure what the status is.)